Without limiting the scope of the invention, its background is described in connection with polysaccharide storage diseases.
Glycogen-storage disease type II (“GSD II” or “Pompe's disease”), is a genetic disorder in humans resulting from the deficiency of acid alpha-glucosidase (“acid maltase”), a lysosomal hydrolase enzyme. The disease is characterized by the abnormal accumulation of glycogen in the lysosomes. (Hirschhorn, R and Reuser, A J J: Glycogen Storage Disease Type II: Acid α-Glucosidase Deficiency. In: The Metabolic and Molecular Bases of Inherited Diseases, 8th edition, Chapter 135, pp 3389-3420, McGraw-Hill, 2001). Polysaccharide storage diseases have also been reported in other animals, particularly in horses, cattle, and sheep.
With respect to humans, three major forms of GSD II have been described: infantile, juvenile, and adult-onset. See J. Ibrahim, et al., Glycogen Storage Disease Type II, http://www.emedicine.com/PED/topic1866.htm (2003). The infantile form of GSD II is described as presenting by 6 months of age and characterized by involvement of cardiac, skeletal and respiratory muscles with rapid progression to death by respiratory and cardiac failure. These inherited conditions are due to an enzyme-deficiency that occurs in the human population at about 1 in 14,000 to 1 in 60,000 live births. The infantile or neonatal form of the disease usually results in death by about twelve to eighteen months of age.
The juvenile (intermediate) form includes infants and children older than 6 months who present with weakness but generally have no cardiac disease. Adult-onset GSD II is slowly progressive and involves progressive muscle weakness that affects the accessory muscles of respiration and finally leads to respiratory failure and death. This form of the disease usually does not involve the heart. The adult-onset form of the disease may appear in the second or third decade of life, and as late as the sixth decade. The disease results in loss of weight and muscle mass. With the loss of muscle mass comes difficulty in breathing because the muscles have difficulty powering the breathing mechanism. The deficient enzyme, acid α-glucosidase, is not required for the vast majority of cellular glycogen because the main pathway for glycogen degradation is not deficient in GSD II disease, energy production is not impaired, and hypoglycemia does not occur. The main issue of deficiency of acid alpha-glucosidase enzymatic activity is the accumulation of structurally normal glycogen in lysosomes and cytoplasm in affected individuals. Excessive glycogen storage within lysosomes interrupts the normal functioning of other organelles and leads to cellular injury and dysfunction of the entire organ involved.
Presently, there is neither a cure for the disease, nor alteration of the clinical course of expected fatality, although some relief has been realized temporarily with high protein diets and with treatment for cardiac and respiratory failure. Alternatively, enzyme replacement therapy with recombinant human acid alpha-glucosidase, (rhGAA), an investigational enzyme replacement therapy for Pompe disease is now in clinical trials (www.genzyme.com and www.pompe.com).
Glycogen and other polysaccharide storage diseases also affect animals other than humans. For example, muscle disease in draft horses has been known for over 100 years, and in 1992 was correlated with polysaccharide storage diseases. Such disorders are commonly referred to as “Monday Morning Disease”, “tying” or “locking up” or simply severe muscle disease have been reported. Draft horses which worked hard six days a week but were given a high grain diet and a day of rest on Sunday were found to be prone to massive muscle injury on Monday. Recently, the polysaccharide storage diseases have been found to be related to problems with giving a draft horse general anesthesia. In addition, glycogen branching enzyme deficiency has been reported as causing muscle weakness in Quarter Horses and related breeds. It is reportedly a separate disorder from polysaccharide storage myopathy and is a deficiency in the enzyme necessary for the formation of normal glycogen. S. Valberg, “Glycogen Storage Disorders of Quarter Horse Foals,” J. Vet. Intern. Med. 15(6): 572-80 (2001).
A dietary solution has been suggested for the polysaccharide storage diseases using a diet with reduced starches and sugars and added fat as an alternative energy source. Providing fats as opposed to carbohydrates as an alternative source of energy to relieve tying up in horses has also been proposed. A subset of horses with chronic exertional rhabdomyolyis have an abnormal accumulation of glycogen (polysaccharide) stores in their muscles and one of the preventative measures suggested is feeding diets without grains and adding a fat supplement to maintain low blood glucose and insulin concentrations. L. Gray, “Polysaccharide Storage Myopathy (Glycogen Storage Disease),” Horse Previews, July 2002. These diets provide only partial relief, if at all, in these affected animals.